During the cutting of glass to a particular configuration, for example, a windshield for a vehicle, numerous finite small edge fractures or leads, as they are known in the art, occur about the perimeter of the glass on the corner edges thereof. These leads offer potential breakage points when the glass undergoes further processing involving thermal and mechanical stress changes. Furthermore, the sharp edges represent a safety hazard during the manual handling of the glass pieces.
In most processes a small portion of the glass material at the upper and lower sharp corners around the perimeter of the glass is removed thus creating generally beveled edges. This procedure, known in the art as glass seaming, increases the fracture resistance of the glass and reduces injuries caused by handling.
Heretofore, the most common method of seaming a piece of glass was primarily manual in nature and oftentimes inaccurate, inefficient and dangerous. In the usual process, two driven, vertically oriented, abrasive grinding belts are positioned adjacent and angular to each other forming a V at their intersection. The edge of a piece of glass to be seamed is then manually held against the V by a workman who attempts to move the glass so that the entire periphery is seamed. Not only is the operator in constant contact with the glass representing an injury hazard, but also such a process has its inherent inaccuracies. First, the operator must assure that the glass is held tightly and perfectly horizontal at all times to create a uniform seam on both sides of the glass. Next, the force applied by the operator as he is holding the glass against the belt must be constant to assure a uniform seam depth. Moreover, when the periphery of the glass is of an irregular shape, such as a windshield, overwhelming operator concentration is required to assure that the seam is consistent around the entire periphery.
The only attempts at automating the seaming process have not been very successful or practical. In one attempt the profile of the glass to be seamed is programmed into a controller for a grinder so that as the glass would move down a conveyor it would stop and the programmed grinder would move around the periphery thereof. This process was not only slow but also required precise positioning and holding of the glass at all times and further required total and exact uniformity of profile from piece to piece. Additionally, the process resulted in extended down times due to lengthy set up procedures when the grinders had to be reprogrammed to seam pieces of a different profile.
Another attempt at automation involved the use of a mechanical sensor associated with the grinder to try to ascertain the position of the edge of the glass as it moved along a conveyor. This sensor, in the form of a reed or finger, would, in theory, ride along the edge of the glass and as the contour of the glass changed, it would mechanically change the position of the grinder accordingly. However, this device too required uniformity in glass configuration and positioning for if the glass were too much off line on the conveyor, the mechanical sensor would be inoperative, totally missing the edge of the glass. Furthermore, the reed fingers were quite susceptible to breakage and as such, the speed of the operation had to be kept quite low. By operating at such a low speed, the time delay between the location of the finger and the location of the grinder induced inaccuracies into the seaming process.
Thus, even with the attempts at automation, today the best method known for glass seaming is still the manual method even though it too is fraught with problems.